Hydrostatic bearing for linear motion guidance

ABSTRACT

A self-compensating hydrostatic (pressurized fluid film) linear bearing that maintains a fluid gap between a carriage and a rail when relative forces are applied. The geometric shape of the rail and mating carriage enable the bearing to have very high stiffness and load capacity without exessive detrimental carriage deformation. The carriages contain bearing grooves and lands which control and use fluid pressure to provide a very high degree of restoring force in response to changes in the fluid gap. The fluid emanating from the bearing gap is prevented from immediately leaking from the bearing carriage, and is instead routed back to the source from which it is pumped, thereby sealing the bearing carriage and simplifying the handling of the lubricating fluid. The hydrostatic bearing is particularly designed to be compact and to be bolt-for-bolt compatible with conventional linear bearings.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional PatentApplication No. 60/406,933 filed on Aug. 30, 2002. The entire contentsof that application are incorporated into this application by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to mechanical bearings and, moreparticularly, to hydrostatic bearings for linear motion guidance.

[0004] 2. Description of Related Art

[0005] A linear bearing typically includes a carriage and a railslideably mounted on the carriage. A component, such as a moveableportion of a machine tool, is typically removably mounted on thecarriage for sliding movement along the rail with the carriage. Aconventional linear bearing uses rolling elements or polymer linings toreduce friction between the carriage and rail.

[0006] In a hydrostatic linear bearing, lubricating fluid is pumped intothe carriage and rail at high pressures so that a thin film of lubricantis maintained between the carriage and rail as the carriage slides alongthe rail, even when large loads are applied to the carriage and rail.The lubricating fluid flows into shallow cavities and channels providedin the carriage and rail. These cavities in the carriage and rail aresometimes referred to as bearing pockets.

[0007] In order to maintain the thin fluid film between the carriage andthe rail, some fluid flow resistance or compensation must be provided inthe bearing. Typically, capillary tubes, orifices, and control valvesare used to provide the required resistance or compensation. Ahydrostatic bearing may also be of the self-compensating type, in whichresistive lands in the bearing pockets (i.e., planar areas over whichfluid flow is restricted), or other bearing pocket features, are used toprovide the required flow resistance or compensation.

[0008] Hydrostatic bearings a very desirable in a number of applicationsbecause they generally have very high stiffness, high load capacity, lowfriction, no wear, high damping, and resistance to contamination. All ofthese advantages make hydrostatic bearings particularly desirable inmachine tool applications, where linear bearings with high rigidity anddamping capabilities are needed to enable very precise motion that isfree of excessive vibration.

[0009] Despite their advantages, hydrostatic bearings have not beenwidely used in the machine tool industry due to a number of practicalproblems with their installation and use. For example, the typicalcompensating devices, orifices, and control valves are often toodifficult to install properly in machine tools, and may also bedelicate, expensive, or too prone to contamination to provide areasonable useable lifetime. Additionally, the fluid used forlubrication is easily contaminated by chips and coolant used in themachining process. For these reasons, linear bearings based on rollingelements have been used predominantly in the machine tool industry.

SUMMARY OF THE INVENTION

[0010] One aspect of the invention relates to a self-compensatinghydrostatic bearing. The self-compensating hydrostatic bearing includesa bearing rail and a bearing carriage constructed and arranged to bemounted for hydrostatically supported movement on the bearing rail. Thebearing carriage includes a plurality of self-compensating bearing padsprovided on surfaces that oppose the bearing rail. The bearing pads areconstructed and arranged to be in fluid communication with one anotherand with a pressurized fluid source.

[0011] End sealing structures are provided on end portions of thebearing carriage. At least one edge of the end sealing structuresengages the bearing rail to prevent hydrostatic fluid from leaking frombetween the bearing carriage and the bearing rail. Side sealingstructures are provided on side portions of the bearing carriage andextend at least a portion of the length of the bearing carriage. Atleast one edge of the side sealing structure engages the bearing rail toprevent hydrostatic fluid from leaking from between the bearing carriageand the bearing rail.

[0012] The bearing also includes a fluid return system provided withinportions of the bearing carriage that are sealed by the end and sidesealing structures. The fluid return system is constructed and arrangedto route fluid towards the pressurized fluid source.

[0013] Another aspect of the invention relates to a self-compensatinghydrostatic bearing. The bearing includes a bearing rail having at leastone substantially contiguous support surface constructed and arranged tosupport the hydrostatic bearing and a bearing carriage constructed andarranged to be mounted for hydrostatically supported movement on thebearing rail.

[0014] The bearing carriage includes a plurality of self-compensatingbearing pads provided on surfaces that oppose the bearing rail. Thebearing pads are constructed and arranged to be in fluid communicationwith one another and with a pressurized fluid source. Sealing structureis provided on portions of the bearing carriage. At least one edge ofthe sealing structure engages the bearing rail to prevent hydrostaticfluid from leaking from between the bearing carriage and the bearingrail. The bearing carriage also includes a fluid return system providedwithin portions of the bearing carriage that are sealed by the sealingstructure. The fluid return system is constructed and arranged to routefluid towards the pressurized fluid source.

[0015] A further aspect of the invention relates to a bearing carriagethat comprises one or more bearing pads and a fluid recovery system. Thebearing pads are constructed and arranged to receive fluid from apressurized fluid source and to cause that fluid to flow selectivelyover a collection of bearing grooves and resistive lands so as to createa supporting fluid layer between the bearing carriage and a structure onwhich the bearing carriage is mounted for movement.

[0016] The fluid recovery system is constructed and arranged to preventfluid from flowing out of the space between the bearing carriage and thestructure on which the bearing carriage is mounted for movement and toroute the fluid back towards the pressurized fluid source. The fluidrecovery system includes sealing structure having contiguous end andside portions. The end portions are constructed and arranged to sealends of the bearing carriage and the side portions are constructed andarranged to extend along at least a portion of sides of the bearingcarriage to seal the sides. The end portions include a double-lippedseal. A first lip of the double-lipped seal engages the structure onwhich the bearing carriage is mounted for movement and the second lip ofthe double-lipped seal prevents debris from entering the bearingcarriage. The fluid recovery system also includes reservoir structuredefined by portions of the bearing carriage and sealed by the sealingstructure and drain grooves constructed and adapted to conductpressurized fluid from the bearing pads to the reservoir structures.

[0017] Further aspects of the invention relate to machine tools orportions thereof mounted on hydrostatic bearings.

[0018] Yet another aspect of the invention relates to a bearingcarriage. The bearing carriage comprises one or more bearing padsconstructed and arranged to receive fluid from a pressurized fluidsource and to cause that fluid to flow selectively over a collection ofbearing grooves and resistive lands so as to create a supporting fluidlayer between the bearing carriage and a structure on which the carriageis mounted for movement.

[0019] The bearing carriage also includes a fluid recovery systemconstructed and arranged to prevent fluid from flowing out of the spacebetween the bearing and the structure on which the bearing carriage ismounted for movement and to route the fluid back towards the pressurizedfluid source. The fluid recovery system includes a sealing structurehaving contiguous end and side portions. The end portions areconstructed and arranged to seal ends of the bearing carriage. The sideportions are constructed and arranged to extend along at least a portionof the sides of the bearing carriage to seal the sides. The end portionsinclude a double-lipped seal. A first lip of the double lipped sealengages the structure on which the bearing carriage for movement, and asecond lip of the double-lipped seal prevents debris from entering thebearing carriage.

[0020] The bearing carriage also includes reservoir structures definedby portions of the bearing carriage and sealed by the sealing structureand drain grooves constructed and arranged to conduct pressurized fluidfrom the bearing pads to the reservoir structures.

[0021] Another further aspect of the invention relates to a hydrostaticbearing. The hydrostatic bearing comprises a bearing rail and a bearingcarriage constructed and arranged to be mounted for hydrostaticallysupported movement on the bearing rail. The bearing carriage includesone or more bearing pads provided on surfaces that oppose the bearingrail. The bearing pads are constructed and arranged to be in fluidcommunication with a pressurized fluid source.

[0022] The bearing carriage also includes seal receiving grooves and asealing structure having contiguous end and side portions. At least aportion of the sealing structure is adapted to be received in the sealreceiving grooves. End portions of the sealing structure includedouble-lipped seals.

[0023] A fluid return system is also included in the bearing carriage.The fluid return system includes a plurality of drain grooves in fluidcommunication with the bearing pads. At least some of the plurality ofdrain grooves are positioned between the bearing pads and the sideportions of the sealing structure.

[0024] Yet another further aspect of the invention relates to a methodof sealing a hydrostatic bearing carriage. The method comprises causingor allowing hydrostatic fluid to flow from hydrostatic bearing padsprovided in the bearing into drain grooves provided along the sides ofthe bearing carriage. The method also involves preventing leakage fromthe drain grooves by positioning sealing structures along the sides ofthe bearing carriage so as to capture hydrostatic fluid flowing out fromthe drain grooves, collecting the hydrostatic fluid in a reservoirprovided as a portion of the hydrostatic bearing carriage, preventingthe hydrostatic reservoir from leaving the reservoir except throughdesignated outlets using a first portion of an end sealing structure,and preventing debris from entering the bearing carriage using a secondportion of the end sealing structure.

[0025] An additional aspect of the invention relates to a hydrostaticbearing pad. The hydrostatic bearing pad comprises a compensatinggroove, an adjacent pocket groove enclosing a first planar area therein,and a second planar area interposed between the compensating groove andthe adjacent pocket groove. The first and second planar areas areconstructed and arranged to resist the flow of pressurized fluid whenthe bearing pad is in a load supporting position relative to anothersurface. The bearing pad does not include grooves between thecompensating groove and the pocket groove.

[0026] Other additional aspects of the invention relate toself-compensating hydrostatic bearings having bearing pads as describedin the preceding paragraph.

[0027] These and other aspects, features and advantages of the inventionwill be described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention will be described with reference to the followingdrawing figures, in which like numerals represent like featuresthroughout the figures, and in which:

[0029]FIG. 1 is a perspective view of a hydrostatic bearing inaccordance with the invention without end caps or seals installed;

[0030]FIG. 2 is a side elevational view of the carriage of FIG. 1;

[0031]FIG. 3 is a schematic diagram of the vertical bearing pads in thecarriage of FIG. 1;

[0032]FIG. 4 is a fluid circuit diagram showing the resistances of thebearing pads of FIG. 3;

[0033]FIG. 5 is a schematic diagram of the horizontal bearing pads ofthe carriage of FIG. 1;

[0034]FIG. 6 is a fluid circuit diagram showing the resistances of thebearing pad of FIG. 5;

[0035]FIG. 7 is another perspective view of the hydrostatic bearing ofFIG. 1, with end caps and seals installed;

[0036]FIG. 8 is a sectional view through Line 8-8 of FIG. 7 showing thereservoir end caps and end seals of the hydrostatic bearing;

[0037]FIG. 9 is a close-up sectional view of a portion of the structureshown in FIG. 8, showing the end caps and seals in more detail;

[0038]FIG. 10 is a sectional elevational view of the carriage of FIG. 1illustrating the side seals;

[0039]FIG. 11 is a close-up sectional view of a portion of the structureshown in FIG. 10 in more detail;

[0040]FIG. 12 is a side elevational view showing a machine tool tablesupported on several hydrostatic bearings of the type shown in FIG. 1;

[0041]FIG. 13 a perspective view showing the underside of the bearingcarriage of FIG. 1;

[0042]FIGS. 14 and 15 are perspective views of the keeper portions ofthe bearing carriage of FIG. 1;

[0043]FIG. 16 is a perspective view of the side and end seals of thebearing carriage of FIG. 1 in isolation without the bearing carriageitself;

[0044]FIG. 17 is a close-up perspective view of a portion of the sideand end seals shown in FIG. 16, illustrating the engagement of the sideand end seals; and

[0045]FIG. 18 is a schematic perspective view of several hydrostaticbearings according to the invention connected to a hydraulic power unit.

DETAILED DESCRIPTION

[0046]FIG. 1 is a perspective view of a hydrostatic linear bearing,generally indicated at 10, according to the present invention. Thebearing 10 is comprised of a carriage 12 that is mounted for sliding,hydrostatically supported movement along a rail 14. The direction ofmovement is shown by arrow M in FIG. 1.

[0047] In the embodiment shown in FIG. 1, the rail 14 has a “T shaped”cross section. The carriage 12 has a central portion 16 and two keepers,18A, 18B that are clamped or bolted to the central portion 16 of thecarriage 12. Alternatively, the carriage 12 may be fabricated as asingle-piece structure; however, the use of the two separable keepers18A, 18B makes the carriage 12 easier to fabricate, and, in particular,easier to finish grind. If the carriage 12 is fabricated as asingle-piece structure, special finish grinding equipment may need to beused.

[0048] The carriage 12 also includes a number of drain grooves 106,108A, 108B, 110A, 110B, 112A, and 112B extending substantially theentirety of the length of the carriage. The drain grooves 106, 108A,108B, 110A, 110B, 112A, and 112B will be described in more detail below.

[0049] The carriage 12 and rail 14 have rectilinear cross-sections inthis embodiment of the invention. (The term “rectilinear,” as usedherein, refers to any shape comprised of line segments withoutsubstantial curvature between adjacent segments.) Although rectilinearcross sectional shapes are generally preferred because they are easierto machine, the carriage and rail of a hydrostatic bearing according tothe invention may have any desired cross sectional shape. Moregenerally, the carriage 12 may be shaped to engage a rail ofsubstantially any cross-sectional shape.

[0050] As shown in FIG. 1, the rail 14 includes drilled and counterboredholes 20 that are used to secure it to a machine tool bed or other rigidstructure. The carriage 12 includes drilled and tapped holes 22 suchthat raised surfaces 24A, 24B, 24C may be clamped rigidly to the matingsurface of a machine tool table or other structure that requires linearmotion guidance. (The use of the hydrostatic bearing 10 will bedescribed in more detail below.)

[0051] In general, the overall size and shape of the carriage 12 andrail 14, and the locations of the holes 20, 22 in the rail and carriagemay be selected so as to be “bolt-for-bolt” compatible with and of thesame size as standard rolling element linear bearings. It isadvantageous if this type of compatible configuration is used, because ahydrostatic bearing 10 according to the invention may then be directlysubstituted for a rolling element-type linear bearing in an existingmachine tool or tool design.

[0052]FIG. 2 is a side elevational view of the carriage 12. The carriage12 is hydrostatically supported by a number of bearing pads provided ininterior surfaces of the carriage 12. The locations of the verticalbearing pads 26A, 26B, 28A, 28B and the horizontal bearing pads 30A, 30Bare also shown in the perspective views of FIGS. 13-15 and will bedescribed in more detail below with respect to those figures. (The terms“vertical” and “horizontal,” as used with respect to the bearing pads,refer to the direction of the applied loads that the respective bearingpads resist.) Fluid pressure exerted through the bearing pads 26A, 26B,28A, 28B maintains the bearing carriage 12 at a small distance from thebearing rail 14. Typically, the clearance between the bearing pads 26A,26B, 28A, 28B, 30A, 30B and the rail 14 would be on the order of about0.001 inches to about 0.005 inches.

[0053] In this description, the terms “fluid” and “hydrostatic fluid”are used interchangeably to refer to any fluid that may be used in abearing 10 according to the present invention. Many such fluids areknown in the art, including hydrocarbon-based oils, silicone-based oils,water, water-based compositions, and air or another suitable gas. Inmachine tool applications, hydrocarbon-based oils may be preferred forsome applications. These oils tend to reduce or eliminate corrosionproblems, and may also have relatively high viscosities, which help toreduce the bearing flow rate and associated pumping power needed topressurize the bearing 10.

[0054] Water-based hydrostatic fluids also have certain advantages andmay also serve in hydrostatic bearings 10 according to the invention.One advantage of water-based hydrostatic fluid is that if machiningcoolant (typically a water-based composition) leaks into or mixes withthe hydrostatic fluid, it may not present a serious contaminationproblem. Water-based hydrostatic fluids may also be used in bearings 10that are produced for the food industry, because of the reduced risk ofcontaminating the consumable product. Additionally, water-based fluidsgenerally have high thermal conductivities, which enables the heatgenerated by the pumping process to be removed much more easily.

[0055]FIG. 3 is a schematic diagram of the vertical pads 26A and 28A,showing their basic geometry and illustrating the route fluid takesthrough the bearing pads 26A, 28A. Vertical bearing pad 26B is similarin design to pad 26A and is therefore not shown. Vertical bearing pad28B is identical in design to 28A and is therefore not shown. In thefollowing description, it is assumed that the fluid path is the same inthe non-illustrated bearing pads 26B, 28B. However, as those of ordinaryskill in the art will realize, the design of the various verticalbearing pads 26A, 26B, 28A, 28B need not be identical.

[0056] A lubricating fluid is pressurized and supplied by pump 32 to theupper and lower bearing pads 26A and 28A. (The details of the hydraulicsupply of bearings 10 according to the invention will be described belowwith respect to FIG. 18.) The fluid enters the lower pad 28A at supplygroove 34 which has a depth sufficient to allow free flow of fluidwithin it. Some fluid crosses leakage lands 36A and 36B, which are at atight gap distance from rail 14, and exits bearing pad 28A. Some fluidcrosses land 38 and enters pocket groove 40. Some fluid also crossescompensating land 42 which is at a small distance from the rail 14; thistight gap creates a pressure drop as the fluid enters compensator groove44. Some fluid leaks from compensator groove 44 across lands 46A and 46Band exits bearing pad 26A. Some fluid is routed from compensator groove44 to pocket groove 48 of bearing pad 26A. Some fluid leaks out ofpocket groove 48 across lands 50A, 50B, and 50C where it exits bearingpad 26A. Fluid is free to flow in the tight gap region between rail 14,and central bearing pad 52, at a pressure that is equal to the fluidpressure in pocket groove 48. Fluid is also supplied at supply pressurefrom pump 32 to the supply grooves 54A and 54B of pad 26A. Some fluidleaks across lands 56A, 56B, 56C, and 56D and exits the bearing pad 26A.Some fluid crosses from supply grooves 54A and 54B across lands 58A and58B to pocket groove 48. Some fluid crosses from supply grooves 54A and54B across compensator lands 60A and 60B to compensator groove 62. Somefluid leaks from compensator groove 62, crosses land 64 and exitsbearing pad 26A. Some fluid is routed from compensator groove 62 tobearing pad 28A where it enters pocket groove 40. Some fluid then flowsfrom pocket groove 40 across lands 66A, 66B, and 66C where it exitsbearing pad 28A. Fluid can flow between compensator groove 44 and pocketgroove 40 but is largely restricted from doing so by land 68. Fluid canflow between compensator groove 62 and pocket groove 48 but is largelyrestricted from doing so by land 70.

[0057] Grooves 54A, 54B, 62, 48, 34, 44, and 40 all should have a depththat is at least about three times larger than the clearance between thepads 28A and 26A and the rail 14 to ensure uniform pressure within eachof these grooves. In the case of grooves 48 and 40, uniform pressure isdesired to spread the load-supporting pressure over the entire pocketarea. In the case of grooves 54A, 54B, 62, 34, and 44, uniform pressureis desired in order to yield the proper hydraulic resistance on theadjacent lands so that the pressure in the respective bearing areas canbe adequately controlled.

[0058] Pad 26A should be fabricated such that lands 52, 50A, 50B, 50C,60A, 60B, 56A, 56B, 56C, 56D, 64, 58A, 58B, and 70 are preferably all onthe same plane and at the same tight gap distance to rail 14. Pad 28Ashould be fabricated such that lands 66A, 66B, 66C, 42, 46A, 46B, 36A,36B, 38, and 68 are preferably all on the same plane and at the sametight gap distance to rail 14.

[0059]FIG. 4 is a fluid circuit diagram of the vertical bearing pads 26Aand 28A (which are identical to the counterpart vertical bearing pads26B and 28B). The various lands described above with respect to FIG. 3are shown in FIG. 4 as circuit resistors. The values of the landresistances, which can be calculated by those skilled in the art offluid dynamics, is dependent upon the fluid viscosity, the length andwidth of the lands, and the clearance between each land and the rail 14.The fluid circuit shown in FIG. 4 can be solved by those skilled in theart of circuit analysis to compute the pressure in each of the bearinggrooves. These pressures may then be multiplied by the correspondingbearing areas to yield the overall vertical force developed by thebearing.

[0060] In order to evaluate how the bearing force changes in response toa change in vertical position of the carriage 12 with respect to therail 14, the fluid gap between the carriage 12 and the rail 14 that wasused to calculate the land resistances would be changed and the analysisdescribed above would be repeated with the new fluid gap data. Acomputer program could be used to carry out this repetitive analysis.Although the bearing pad geometries may be chosen to suit particularapplications of the hydrostatic bearing 10, it is preferable if the thebearing groove and land geometry are optimized to provide very highbearing stiffness and load capacity in the vertical direction with theminimum possible flow rate of fluid through the bearing 10 because highfluid flow rates typically require great amounts of pumping power.

[0061] Grooves 54A, 54B, 62, 48, 34, 44, and 40 are shown in FIG. 3 withrounded corners; however, they may be fabricated with sharp squarecorners or another geometric profile without considerable effect onbearing operation, since the hydraulic resistances of the adjacent landswill change by a very small percentage of their overall resistancevalues.

[0062] As shown in FIG. 3 and described above, fluid is routed betweenpad 28A and pad 26A in two places, from compensator groove 44 to pocketgroove 48, and from compensator groove 62 to pocket groove 40. Thesefluid transfers may be accomplished by the use of drilled holes incarriage 12 and keeper 18A, or they may be accomplished with the use ofrigid tubing external to carriage 12. Similarly, fluid may be routed atsupply pressure from pump 32 to supply grooves 34, 54A, and 54B with theuse of external tubing followed by holes drilled in carriage 12 andkeeper 18A.

[0063]FIG. 5 is a schematic view of the horizontal bearing pads 30A and30B, showing their basic geometry and illustrating the route that fluidtakes through the bearing pads 30A, 30B. A lubricating fluid ispressurized and supplied by pump 32 to the upper and lower bearing pads30A and 30B. (The same pump 32 may be used to supply the horizontalbearing pads 30A, 30B and the vertical bearing pads 26A, 26B, 28A, 28B,or two different pumps 32 may be used.) The fluid enters pad 30A atsupply groove 72A which is at a depth sufficient to allow free flow offluid within it. Some fluid leaks from supply groove 72A across leakagelands 74A and 76A, which are at a tight gap distance from rail 14, andexits bearing pad 30A. Some fluid flows from supply groove 72A acrosslands 78AA and 80AA to pocket groove 82AA and some flows across lands78AB and 80AB to pocket groove 82AB. Some fluid flows from supply groove72A across compensator lands 84AA, 86AA, 88AA to compensator groove90AA. Some of the fluid which enters compensator groove 90AA leaks to orfrom pocket groove 82AA across land 100AA. The remainder of the fluidwhich enters compensator groove 90AA is routed to bearing pad 30B whereit enters pocket groove 82BA and provides uniform pressure to pocketgroove 82BA before leaking across lands 92BA, 94BA, and 96BA and exitingbearing pad 30B. The fluid in the tight clearance of bearing pad 98BAwill be at a pressure equal to the fluid pressure in pocket groove 82BAbecause pocket groove 82BA completely surrounds bearing pad 98BA. Fluidis also supplied at supply pressure from pump 32 to supply groove 72B ofbearing pad 30B. Some of the fluid which enters supply groove 72B leaksacross lands 74B and 76B and exits bearing pad 30B. Some of the fluidwhich enters supply groove 72B leaks across lands 78BA and 80BA topocket groove 82BA, and some leaks across lands 78BB and 80BB to pocketgroove 82BB. Some of the fluid which enters supply groove 72B leaksacross compensator lands 84BA, 86BA, and 88BA to compensator groove90BA. Some fluid may across land 100BA between compensator groove 90BAand pocket groove 82BA. The remainder of fluid entering compensatorgroove 90BA is routed to pad 30A where it enters pocket grooves 82AA andleaks across lands 92AA, 94AA, and 96AA and exits bearing pad 30A. Thefluid in the tight gap clearance of bearing pad 98AA will be at apressure equal to the fluid pressure in pocket groove 82AA becausepocket groove 82AA completely surrounds bearing pad 98AA. Some of thefluid which enters supply groove 72A leaks across compensator lands84AB, 86AB, and 88AB to compensator groove 90AB. Some fluid may acrossland 100AB between compensator groove 90AB and pocket groove 82AB. Theremainder of fluid entering compensator groove 90AB is routed to pad 30Bwhere it enters pocket groove 82BB and leaks across lands 92BB, 94BB,and 96BB and exits bearing pad 30B. The fluid in the tight gap clearanceof bearing pad 98BB will be at a pressure equal to the fluid pressure inpocket groove 82BB because pocket groove 82BB completely surroundsbearing pad 98BB. Some of the fluid which enters supply groove 72B leaksacross compensator lands 84BB, 86BB, and 88BB to compensator groove90BB. Some fluid may across land 100BB between compensator groove 90BBand pocket groove 82BB. The remainder of fluid entering compensatorgroove 90BB is routed to pad 30A where it enters pocket groove 82AB andleaks across lands 92AB, 94AB, and 96AB and exits bearing pad 30A. Thefluid in the tight gap clearance of bearing pad 98AB will be at apressure equal to the fluid pressure in pocket groove 82AB becausepocket groove 82AB completely surrounds bearing pad 98AB.

[0064] Grooves 82AA, 82AB, 82BA, 82BB, 90AA, 90AB, 90BA, 90BB, 72A, and72B all should have a depth that is at least three times larger than theclearance between the pads 30A and 30B and the rail 14 to ensure uniformpressure within each of these grooves. In the case of grooves 82AA,82AB, 82BA, and 82BB, uniform pressure is desired in order to spread theload-supporting pressure over the entire pocket area. In the case ofgrooves 90AA, 90AB, 90BA, 90BB, 72A, and 72B, uniform pressure isdesired in order to yield the proper hydraulic resistance on theadjacent lands so that the pressure in the respective bearing areas canbe adequately controlled.

[0065] Pad 30A is fabricated such that lands 98AA, 98AB, 84AA, 84AB,86AA, 86AB, 88AA, 88AB, 92AA, 92AB, 94AA, 94AB, 96AA, 96AB, 78AA, 78AB,80AA, 80AB, 100AA, 100AB, 74AA, 74AB, 76AA, 76AB are preferably all onthe same plane and at the same tight gap distance to rail 14. Pad 30Bshould be fabricated such that lands 98BA, 98BB, 84BA, 84BB, 86BA, 86BB,88BA, 88BB, 92BA, 92BB, 94BA, 94BB, 96BA, 96BB, 78BA, 78BB, 80BA, 80BB,100BA, 100BB, 74BA, 74BB, 76BA, 76BB are preferably all on the sameplane and at the same tight gap distance to rail 14.

[0066]FIG. 6 is a schematic diagram showing the fluid resistances of thehorizontal bearing pad 30A. Each of the resistances shown in FIG. 6represents one of the lands of the horizontal bearing pad 5A. The valuesof the resistances of the horizontal bearing pad 30A may be calculatedas was described above with respect to the vertical bearing pads 26A,26B, 28A, 28B.

[0067] Grooves 82AA, 82AB, 82BA, 82BB, 90AA, 90AB, 90BA, 90BB, 72A, and72B are shown in FIG. 5 with rounded corners; however, they may befabricated with sharp square corners or another geometric profilewithout considerable effect on bearing operation since the hydraulicresistances of the adjacent lands will change by a very small percentageof their overall resistance values.

[0068] As shown in FIG. 5, fluid is routed between pad 30A and pad 30Bin four places: from compensator groove 90AB to pocket groove 82BB, fromcompensator groove 90AA to pocket groove 82BA, from compensator groove90BA to pocket groove 82AA, and from compensator groove 90BB to pocketgroove 82AB. As with the fluid transfers in the vertical bearing pads26A, 26B, 28A, 28B, these fluid transfers may be accomplished by the useof drilled holes in carriage 12, or they may be accomplished with theuse of rigid tubing external to carriage 12. Similarly, fluid may berouted at supply pressure from pump 32 to supply grooves 72A and 72Bwith the use of external tubing followed by holes drilled in carriage12.

[0069] In the vertical and horizontal bearing pads shown in FIGS. 3 and5 and described above, lands 58A, 58B, 70, 38, 68, 78AA, 78AB, 78BA,78BB, 80AA, 80AB, 80BA, 80BB, 100AA, 100AB, 100BA, and 100BB allowleakage paths between adjacent compensators, pockets, and supplygrooves. These leakage paths tend to reduce the pressure response of thebearing and therefore reduce its stiffness and load-carrying capability.However, a greater factor that overcomes the effect of these fluidleakage paths is the ability to arrange pocket grooves 48, 40, 82AA,82AB, 82BA, and 82BB such that they are closer to the compensatinggrooves, and, therefore, spread the load-supporting pocket pressuresover a larger area. By better utilizing the available pad area, thebearing pad configurations of the hydrostatic bearing 10 provide higherstiffness and load capacity.

[0070]FIG. 7 is another perspective view of the hydrostatic bearing ofFIG. 1, with its seals and endcaps installed. FIG. 8 is a sectional viewthrough Line 8-8 of FIG. 7, and FIG. 9 is a close-up view of portion A(enclosed in dotted line) of FIG. 8. FIGS. 7-9 show the hydrostaticbearing of FIG. 1 with end caps 102A and 102B attached to carriage 12and keepers 18A and 18B. End caps 102A and 102B contain reservoirs 104Aand 104B (visible in the views of FIGS. 8 and 9) to which the fluidflows into from bearing pads 26A, 26B, 28A, 28B, 30A, and 30B as well asfrom drain grooves 106, 108A, 108B, 11A, 110B, 112A, and 112B. (As wasdescribed above, the drain grooves are provided at the corners of thecarriage 12 and are visible in the views of FIGS. 1 and 2.)Double-lipped end seals 114A and 114B are attached to end caps 102A and102B. The double-lipped end seals 114A, 114B are attached to rigidplates 113A, 113B in order to provide them with additional stiffness.Lips 116 of end seals 114A and 114B are in sliding engagement with rail14 and serve to trap the fluid into reservoirs 104A and 104B and largelyprevent fluid from leaking directly out of the hydrostatic bearing 10.The fluid flows out of reservoirs 104A or 104B through at least onedrain outlet 118A and/or 118B. One or more of the drain outlets 118A,118B may be plugged, but at least one drain outlet 118A, 118B is used toroute the fluid to a hose or tubing assembly, where the fluid isreturned to the hydraulic supply source.

[0071]FIG. 10 is a sectional side elevational view of the hydrostaticbearing 10, illustrating side seals 120A and 120B that are received byacceptor grooves 122A and 122B within keeper portions 18A and 18B of thebearing carriage 12. FIG. 11 is an enlarged sectional view of portion Bof FIG. 10, illustrating the side seals 120A, 120B in more detail. Theside seals 120A, 120B slidingly engage the bearing rail 14, serve totrap fluid, and allow the trapped fluid to be routed through draingrooves 112A and 112B into reservoirs 104A and 104B to prevent fluidfrom leaking directly out of the hydrostatic bearing 10. As shown inFIG. 11, the side seals 120A, 120B have a generally u-shaped portion 121that opens upwardly, towards the top of the drain groove 112A, 112B. Theside seals 120A, 120B are positioned in the acceptor groove 122A, 122Bsuch that one wall of the u-shaped portion 121 of the side seal 120A,120B is in contact with the keeper 18A, 18B and the other wall of theu-shaped portion 121 is in contact with the bearing rail 14.

[0072]FIG. 13 is a perspective view of the underside of the centralportion 16 of the carriage 12 without the keepers 18A, 18B installed.FIG. 13 shows the relative locations and extents of the vertical bearingpads 26A, 26B and the horizontal bearing pads 30A, 30B. FIGS. 14 and 15are perspective views of the keepers 18A and 18B, showing the locationsand extents of vertical bearing pads 28A and 28B on the keepers 18A and18B. The positions of the drain grooves 106, 108A, 108B, 110A, 110B,112A, and 112B and seal acceptor grooves 122A, 122B are also shown.

[0073] Each side of the central portion 16 of the bearing carriage 12has a set of threaded holes 222 provided in respective connectingsurfaces 220A and 220B. A set of complimentary, counterbored throughholes 226 are provided in the keepers 18A and 18B. When the keepers 18Aand 18B and central portion 16 of the carriage 12 are assembled, boltsare inserted through the holes 226 in the keepers 18A, 18B and into thethreaded holes 222 of the central portion 16 of the carriage 12 suchthat the engaging surfaces 220A, 220B of the central portion 16 and theengaging surfaces 224A, 224B of the keepers 18A, 18B are adjacent, asshown in FIG. 10.

[0074] The bearing pad grooves and other surface features shown in FIGS.13-15 may be formed by milling, electrical discharge machining, or otherknown techniques.

[0075]FIG. 16 is a perspective view showing the end seals 114A, 114B andside seals 120A, 120B in isolation. As was described above, the endseals 114A, 114B are constructed of a rubber material molded so as toattach to rigid plates 113A, 113B, for example, steel or aluminumplates, to provide them with greater rigidity. In alternativeembodiments, the end seals 114A, 114B may not be attached to rigidplates 113A, 1133B

[0076] As is shown best in FIG. 17, a close-up perspective view ofportion “C” of FIG. 16, the side seals 120A, 120B are inserted intoreceptacles 115 formed in the end seals 114A, 114B such that they havean interference fit with the receptacles 115. In one embodiment, theside seals 120A, 120B may be made slightly longer than required, suchthat they can be maintained in compression during operation. Inalternative embodiments of the invention, the side seals 120A, 120B andthe end seals 114A, 114B may be molded or cast as a single structure,bonded together, or otherwise caused to adhere to one another to form aunitary structure.

[0077] The bearing pads 26A, 26B, 28A, 28B, 30A, 30B described above aredesigned for a self-compensating hydrostatic bearing. However, those ofordinary skill in the art will realize that the other features of thecarriage 12 and rail 14, including the sealing structures (i.e., the endseals 114A, 114B and side seals 120A, 120B) and the drain grooves 106,108A, 108B, 110A, 110B, 112A, and 112B may be used without theparticular bearing pads 26A, 26B, 28A, 28B, 30A, 30B described above.For example, in alternative embodiments of the invention, a carriagehaving end seals, side seals and a drain groove arrangement similar tothat described above could be used with bearing pads that are notself-compensating. Bearing pads that are not self-compensating could usecapillary tubes or valves for compensation purposes, as one of ordinaryskill in the art will readily be able to appreciate.

[0078] Conversely, the self-compensating bearing pads 26A, 26B, 28A,28B, 30A, 30B described above may be used on other types ofhydrostatically supported devices and in other types of fluidstaticbearings without the other features described herein.

[0079]FIG. 18 is a schematic perspective view illustrating four bearingcarriages 12 riding on two carriage rails 14. In general, severalbearing carriages 12 may be provided on the same carriage rail 14,particularly if those bearing carriages 12 are fixed in position withrespect to one another (e.g., by being bolted to the bed of a machinetool, as will be described below). Alternatively, several shortersegments of bearing rail 14 could be provided, one segment for eachbearing carriage 12.

[0080]FIG. 18 also illustrates the details of the hydraulic fluidconnections for the bearings 10 according to the present invention. Ahydraulic power unit 230 delivers hydraulic fluid under high pressurethrough a conduit 232. The hydraulic power unit 230 includes all of thecomponents necessary to deliver temperature controlled fluid that isrelatively free of contaminant particles at high pressure with minimalpressure pulsations. For example, the hydraulic power unit 230 mayinclude a reservoir, a pump, an electric motor, a filter, a pressureregulating valve, a pressure gauge, and a heat rejection system, such asan air-to-oil heat exchanger.

[0081] The conduit 232 from the hydraulic power unit 230 branches suchthat one branch connects with each bearing carriage 12. The branches ofthe conduit 232 are received by a fluid inlets 119 in the end seals114A, 114B of the bearing carriages 12. (Depending on the configurationof the bearings 10, the conduit 232 may connect to a fluid inlet 119 oneither end seal 114A, 114B. The unused fluid inlet 119 may be plugged oromitted.) The connection between the conduit 232 branch and the fluidinlet 119 of the end seal may be any appropriate type of conventionalhydraulic connection. From the fluid inlet 119, the pressurized fluid isdistributed to the supply grooves 34, 54A, 54B by an internal network ofpassageways. Once used, the fluid is collected in the reservoirs 104A,104B and returned via return conduits 238, which connect to the drainoutlets 118A, 1181B and the return portions of the hydraulic power unit232.

[0082]FIG. 12 is a side elevational view of a machine tool 200,illustrating a typical application for a hydrostatic bearing 10according to the present invention. A machine tool table 66 is supportedby four bearing assemblies 10 which ride on two rails 14. Although onlytwo bearing assemblies 10 are shown, at least four are typically used toprovide adequate pitch and yaw stability to table 202. The rails 14 ofthe hydrostatic bearings 10 are horizontally clamped to a machine bed204 using wedges 206A and 206B. The rails 14 are clamped vertically tomachine bed 204 using a plurality of bolts 208 threadedly secured withinmachine bed 204 through counterbored holes 20 provided in the rail 14.Two of the hydrostatic bearings 10 are clamped horizontally to the table202 using wedges 210 (one wedge 210 is shown in the view of FIG. 12).The other two hydrostatic bearings 10 are floated into alignment bypressurizing them with lubricating fluid, thus allowing hydrostaticbearings 10 to float horizontally into a self-aligning position. Oncethe two wedge-secured hydrostatic bearings 10 are in alignment, thebolts that secure them to the table 202 are tightened. Although FIG. 12illustrates the use of wedges 206A, 206B, and 210, many other mechanismsto clamp the rails 14 and the hydrostatic bearings 10 are possible andare within the scope of the invention.

[0083] A hydrostatic bearing 10 may be used in a number of differenttypes of machine tools, and in any other application in which linearmotion guidance is required. However, hydrostatic bearings 10 accordingto the invention may be particularly beneficial when used in lathes. Forexample, hydrostatic bearings 10 may be used in the QUEST® turningmachines manufactured by HARDINGE, Inc. (Elmira, N.Y., United States).Hydrostatic bearings 10 may also be useful in grinding machines, millingmachines, boring machines, and other machine tools in which acombination of high stiffness and damping are beneficial.

[0084] A hydrostatic bearing 10 according to the present invention mayhave certain advantageous performance characteristics. For example, ahydrostatic bearing 10 according to the invention would typically havehigh static and dynamic stiffnesses. A hydrostatic bearing 10 may alsooperate with very low friction, because the seals described above withrespect to FIGS. 7-11 would generally be the only components creatingfriction. Because the carriage 12 rides on a layer of fluid, and forother reasons, the hydrostatic bearing 10 may have up to ten times theforce damping capabilities of a conventional rolling element linearbearing. Additional advantages may include an essentially unlimitedtranslational (feed) rate, an essentially unlimited fatigue life (withsubstantially no component wear because the carriage 12 and rail 14 arenot in contact), substantially no change in positioning accuracy of amachine tool mounted on hydrostatic bearings 10 over time, substantiallyno damage to the hydrostatic bearing 10 under heavy “crash” loads (i.e.,when the bearing 10 stops suddenly at the ends of its travel range).Moreover, the hydrostatic bearing 10 is self cleaning if fluid flow ismaintained between the carriage 12 and rail continuously 14.

[0085] When installed in a machine tool and used to produce parts, thefeatures of the hydrostatic bearing 10 may also lead to certain otheradvantages. For example, the hydrostatic bearing 10 may improve toollife. Additionally, parts may be produced with better surface finishesand better roundnesses for round parts. A machine tool mounted onhydrostatic bearings 10 may also have improved hard turning capability,improved interrupted cutting capability, and improved positioningaccuracy. Some of the advantages and benefits described above willbecome apparent from the following example.

EXAMPLE 1

[0086] A hydrostatic bearing 10 according to the invention is installedso as to support operational movement in a QUEST® 51 turning machine(Hardinge, Inc., Elmira, N.Y., United States) using the installationprocedure described above. Four hydrostatic bearings 10 according to thepresent invention are installed to guide motion in the X-axis and fourare installed to guide motion in the Z-axis. No adaptations to theturning machine are required in order to accommodate the hydrostaticbearings 10; however, hydraulic hoses are provided for each hydrostaticbearing 10. A two-inch round A2 tool steel blank was prepared with fourslots milled around its circumference for interrupted cutting. It wasthen hardened to 60-62 Rc. The part was then roughed with a {fraction(5/16)} inch diameter round cubic boron nitride (CBN) insert at 450SFM/0.002 ipr/0.030 doc with five passes. Subsequently, the part wasfinished with a 55 degree CBN insert at 550 SFM/0.003 ipr/0.005 doc withone pass, and then threaded with a CBN triangular insert. The surfacefinish of the part was consistently in the 5 to 6 microinch range, animprovement of approximately a factor of two when compared with anidentical part machined on a comparable QUEST® 51 turning machinewithout a hydrostatic bearing. Additionally, the tool life of theinterrupted turning insert was increased by a factor of three whencompared to the life of an insert used on the turning machine withoutthe hydrostatic bearing.

[0087] Although the invention has been described with respect to certainembodiments, those embodiments are intended to be illustrative, ratherthan limiting. Modifications and variations to the invention arepossible, within the scope of the appended claims.

What is claimed is:
 1. A self-compensating hydrostatic bearing,comprising: a bearing rail; a bearing carriage constructed and arrangedto be mounted for hydrostatically supported movement on said bearingrail, said bearing carriage including a plurality of self-compensatingbearing pads provided on surfaces that oppose said bearing rail, saidplurality of self-compensating bearing pads being constructed andarranged to be in fluid communication with one another and with apressurized fluid source; end sealing structures provided on endportions of said bearing carriage, at least one edge of said end sealingstructure engaging said bearing rail to prevent hydrostatic fluid fromleaking from between said bearing carriage and said bearing rail; sidesealing structures provided on side portions of said bearing carriageand extending at least a portion of a length of said bearing carriage,at least one edge of said side sealing structure engaging said bearingrail to prevent hydrostatic fluid from leaking from between said bearingcarriage and said bearing rail; and a fluid return system providedwithin portions of said bearing carriage that are sealed by said end andside sealing structures, said fluid return system being constructed andarranged to route fluid towards said pressurized fluid source.
 2. Thehydrostatic bearing of claim 1, wherein said bearing rail includes atleast one substantially contiguous support surface constructed andadapted to support said hydrostatic bearing.
 3. The hydrostatic bearingof claim 2, wherein said end sealing structures each include adouble-lipped seal.
 4. The hydrostatic bearing of claim 3, wherein saidside sealing structures are contiguous with said end sealing structures.5. The hydrostatic bearing of claim 4, wherein said bearing carriagecomprises a central portion and removably mounted keeper portions. 6.The hydrostatic bearing of claim 5, wherein said side sealing structuresare provided on said keeper portions.
 7. The hydrostatic bearing ofclaim 6, wherein said fluid return system comprises one or more draingrooves in communication with said bearing pads.
 8. The hydrostaticbearing of claim 7, wherein said drain grooves are positioned betweensaid bearing pads and said side sealing structures so as to receivefluid directed from the bearing pads towards said side sealingstructures.
 9. The hydrostatic bearing of claim 7, wherein said fluidreturn system further comprises at least one reservoir in communicationwith said one or more drain grooves, said at least one reservoir beingconstructed and arranged to hold fluid being routed back towards thepressurized fluid source.
 10. The hydrostatic bearing of claim 9,wherein said bearing includes one end sealing structure and onereservoir on each end of said bearing carriage.
 11. The hydrostaticbearing of claim 10, wherein said reservoirs are defined by a portion ofa portion of the ends of said bearing carriage and are sealed by thedouble-lipped seal of said end sealing structure.
 12. The hydrostaticbearing of claim 11, wherein said end sealing structures include fluidinlet ports constructed and arranged to receive hydraulic fluid hoses,said fluid input ports being in fluid communication with said bearingpads.
 13. The hydrostatic bearing of claim 12, wherein said end sealingstructures include fluid outlet ports constructed and arranged toreceive hydraulic fluid hoses, said fluid outlet ports being in fluidcommunication with said reservoirs.
 14. The hydrostatic bearing of claim13, wherein said rail and said bearing carriage have rectilinear shapes.15. The hydrostatic bearing of claim 14, wherein said drain grooves areprovided at corners of said bearing carriage.
 16. The hydrostaticbearing of claim 1, wherein said bearing rail has a T-shapedcross-sectional area.
 17. The hydrostatic bearing of claim 1, furthercomprising one or more fastening holes provided on upper surfaces ofsaid bearing carriage, said fastening holes being constructed andarranged to allow a machine component to be removably mounted on saidbearing carriage.
 18. A machine tool mounted on one or more hydrostaticbearings according to claim
 1. 19. A self-compensating hydrostaticbearing, comprising: a bearing rail including at least one substantiallycontiguous support surface constructed and arranged to support saidhydrostatic bearing; a bearing carriage constructed and arranged to bemounted for hydrostatically supported movement on said bearing rail,said bearing carriage including a plurality of self-compensating bearingpads provided on surfaces that oppose said bearing rail, said pluralityof self-compensating bearing pads being constructed and arranged to bein fluid communication with one another and with a pressurized fluidsource; sealing structure provided on portions of said bearing carriage,at least one edge of said sealing structure engaging said bearing railto prevent hydrostatic fluid from leaking from between said bearingcarriage and said bearing rail; and a fluid return system providedwithin portions of said bearing carriage that are sealed by said sealingstructure, said fluid return system being constructed and arranged toroute fluid towards said pressurized fluid source.
 20. The hydrostaticbearing of claim 19, wherein said sealing structure comprises contiguousside and end sealing portions.
 21. The hydrostatic bearing of claim 20,wherein said end sealing portion comprises a double-lipped seal.
 22. Thehydrostatic bearing of claim 21, wherein said bearing carriage comprisesa central portion and removably mounted keeper portions.
 23. Thehydrostatic bearing of claim 22, wherein said side sealing portions areprovided on said keeper portions.
 24. The hydrostatic bearing of claim23, wherein said fluid return system comprises one or more drain groovesin communication with said bearing pads.
 25. The hydrostatic bearing ofclaim 24, wherein said drain grooves are positioned between said bearingpads and said side sealing structures so as to receive fluid directedfrom the bearing pads towards said side sealing structures.
 26. Thehydrostatic bearing of claim 25, wherein said fluid return systemfurther comprises at least one reservoir in communication with said oneor more drain grooves, said at least one reservoir being constructed andarranged to hold fluid being routed back towards the pressurized fluidsource.
 27. The hydrostatic bearing of claim 26, wherein one end sealingportion and one reservoir are provided on each end of said bearingcarriage.
 28. The hydrostatic bearing of claim 27, wherein saidreservoirs are defined by a portion of the ends of said bearing carriageand are sealed by the double-lipped seal of said end sealing portion.29. The hydrostatic bearing of claim 28, wherein said end sealingportions include fluid inlet ports constructed and arranged to receivehydraulic fluid hoses, said fluid input ports being in fluidcommunication with said bearing pads.
 30. The hydrostatic bearing ofclaim 29, wherein said end sealing portions include fluid outlet portsconstructed and arranged to receive hydraulic fluid hoses, said fluidoutlet ports being in fluid communication with said reservoirs.
 31. Thehydrostatic bearing of claim 30, wherein said rail and said bearingcarriage have rectilinear shapes.
 32. The hydrostatic bearing of claim31, wherein said drain grooves are provided at corners of said bearingcarriage.
 33. The hydrostatic bearing of claim 19, further comprisingone or more fastening holes provided on upper surfaces of said bearingcarriage, said fastening holes being constructed and arranged to allow amachine component to be removably mounted on said bearing carriage. 34.The hydrostatic bearing of claim 19, wherein said bearing rail has aT-shaped cross-sectional area.
 35. A machine tool mounted on one or morehydrostatic bearings according to claim
 19. 36. A bearing carriage,comprising: one or more bearing pads constructed and arranged to receivefluid from a pressurized fluid source and to cause that fluid to flowselectively over a collection of bearing grooves and resistive lands soas to create a supporting fluid layer between said bearing carriage anda structure on which said bearing carriage is mounted for movement; anda fluid recovery system constructed and arranged to prevent fluid fromflowing out of the space between said bearing carriage and the structureon which said bearing carriage is mounted for movement and to route thefluid back towards the pressurized fluid source, said fluid recoverysystem comprising: a sealing structure having contiguous end and sideportions, said end portions being constructed and arranged to seal endsof said bearing carriage and said side portions being constructed andarranged to extend along at least a portion of sides of said bearingcarriage to seal said sides, said end portions including a double-lippedseal, a first lip of the double-lipped seal engaging the structure onwhich said bearing carriage is mounted for movement and a second lip ofsaid double-lipped seal preventing debris from entering said bearingcarriage; reservoir structures defined by portions of said bearingcarriage and sealed by said sealing structure; and drain groovesconstructed and arranged to conduct pressurized fluid from said bearingpads to said reservoir structures.
 37. The bearing carriage of claim 36wherein said drain grooves are between said bearing pads and the sideportions of said sealing structure.
 38. The bearing carriage of claim37, wherein the structure on which said bearing carriage is mounted formovement is a bearing rail having a shape complimentary to that of thebearing carriage.
 39. The bearing carriage of claim 38, wherein thebearing rail has a T-shaped cross-sectional area.
 40. The bearingcarriage of claim 39, wherein said bearing carriage is comprised of acentral portion and removably mounted keeper portions.
 41. The bearingcarriage of claim 40, wherein the side portions of said sealingstructure are received in grooves provided in said keeper portions. 42.The bearing carriage of claim 41, wherein the side portions of saidsealing structure have a substantially upwardly-facing u-shapedcross-section.
 43. The bearing carriage of claim 36, wherein the endportions of said sealing structure include fluid inlet and outlet ports,said fluid inlet ports being in fluid communication with said bearingpads and said fluid outlet ports being in communication with saidreservoir structures.
 44. A hydrostatic bearing comprising: a bearingrail; and a bearing carriage constructed and arranged to be mounted forhydrostatically supported movement on said bearing rail, said bearingcarriage including one or more bearing pads provided on surfaces thatoppose said bearing rail, said one or more bearing pads beingconstructed and arranged to be in fluid communication with a pressurizedfluid source; seal receiving grooves; a sealing structure havingcontiguous side and end portions, at least a portion of said sealingstructure being adapted to be received in the seal receiving grooves ofsaid bearing carriage, end portions of said sealing structure includingdouble-lipped seals; a fluid return system including a plurality ofdrain grooves in fluid communication with said one or more bearing pads,at least some of said plurality of drain grooves being positionedbetween the bearing pads and the side portions of said sealingstructure.
 45. The hydrostatic bearing of claim 45, wherein said bearingcarriage further comprises one or more reservoirs in fluid communicationwith said plurality of drain grooves.
 46. The hydrostatic bearing ofclaim 45, wherein said reservoirs are provided in end portions of saidbearing carriage.
 47. The hydrostatic bearing of claim 44, wherein saidbearing rail has a rectilinear shape.
 48. The hydrostatic bearing ofclaim 47, wherein said bearing rail has a T-shaped cross-sectional area.49. The hydrostatic bearing of claim 44, wherein said one or morebearing pads are self-compensating bearing pads.
 50. A method of sealinga hydrostatic bearing carriage, comprising: causing or allowinghydrostatic fluid to flow from hydrostatic bearing pads provided in thebearing carriage into drain grooves provided along the sides of thebearing carriage; preventing leakage from the drain grooves bypositioning sealing structures along the sides of the bearing carriageso as to capture hydrostatic fluid flowing out from the drain grooves;collecting the hydrostatic fluid in a reservoir provided as a portionthe hydrostatic bearing carriage; preventing the hydrostatic fluid fromleaving the reservoir except through designated outlets using a firstportion of an end sealing structure; and preventing debris from enteringthe bearing carriage using a second portion of the end sealingstructure.
 51. A hydrostatic bearing pad, comprising: a compensatinggroove; an adjacent pocket groove enclosing therein a first planar areaconstructed and arranged to resist a flow of pressurized fluid when saidhydrostatic bearing pad is in a load supporting position relative toanother surface; and a second planar area interposed between saidcompensating groove and said pocket groove, said planar area beingconstructed and arranged to resist the flow of the pressurized fluidfrom said compensating groove to said adjacent pocket groove when saidbearing pad is in the load supporting position relative to the othersurface; wherein said bearing pad does not include grooves between thecompensating groove and the pocket groove.
 52. The hydrostatic bearingpad of claim 51, further comprising a supply groove proximate to saidcompensating groove, said supply groove and said compensating groovebeing separated by a third planar area that is constructed and arrangedto resist the flow of pressurized fluid from said supply groove to saidcompensating groove.
 53. A self-compensating hydrostatic bearing,comprising: a bearing rail; a bearing carriage constructed and arrangedto be mounted for hydrostatically supported movement on said bearingrail, said bearing carriage including a plurality of bearing pads, onesof said plurality of bearing pads including a compensating groove; anadjacent pocket groove enclosing therein a first planar area constructedand arranged to resist a flow of pressurized fluid when said bearing padis in a load supporting position relative to said bearing rail; and asecond planar area interposed between said compensating groove and saidpocket groove, said planar area being constructed and arranged to resistthe flow of the pressurized fluid from said compensating groove to saidadjacent pocket groove when said bearing pad is in the load supportingposition relative to the other surface; wherein the ones of saidplurality of bearing pads do not include grooves between thecompensating groove and the pocket groove; drain grooves extending alongthe length of said bearing carriage, said drain grooves beingconstructed and arranged to receive fluid flowing from said plurality ofbearing pads; and sealing structure having side and end portions, theside portions of said sealing structure being constructed and arrangedto sealingly engage said bearing rail to prevent fluid in said draingrooves from flowing out of said drain grooves.
 54. The hydrostaticbearing of claim 53, wherein the drain grooves are in fluidcommunication with reservoirs provided in end portions of said bearingcarriage, said reservoirs including fluid inlet and outlet ports incommunication with a hydraulic power unit.
 55. The hydrostatic bearingof claim 54, wherein the reservoirs are sealed by the end portions ofsaid sealing structure.
 56. The hydrostatic bearing of claim 53, whereinthe end portions of said sealing structures include double-lipped seals.57. A hydrostatic bearing, comprising: a bearing rail having asubstantially T-shaped cross-sectional area; a bearing carriageconstructed and arranged to engage said bearing rail, said bearingcarriage having one or more bearing pads, each of said one or morebearing pads including a first groove; an adjacent pocket grooveenclosing therein a first planar area constructed and arranged to resista flow of pressurized fluid when said bearing carriage is in a loadsupporting position relative to said bearing rail; and a second planararea interposed between said first groove and said pocket groove, saidplanar area being constructed and arranged to resist the flow of thepressurized fluid from said first groove to said adjacent pocket groovewhen said bearing pad is in the load supporting position relative to theother surface; said bearing carriage further comprising drain groovesconstructed and arranged to receive fluid from said one or more bearingpads and direct the fluid towards a fluid return; wherein said one ormore bearing pads do not include drain grooves between said first grooveand said adjacent pocket groove.
 58. The hydrostatic bearing of claim57, wherein the hydrostatic bearing is self-compensating.
 59. Thehydrostatic bearing of claim 57, wherein said bearing carriage furthercomprises separable keeper portions that engage portions of said bearingrail.
 60. The hydrostatic bearing of claim 59, wherein said keeperportions include sealing structures mounted within seal grooves.
 61. Thehydrostatic bearing of claim 60, wherein said drain grooves are providedin said keeper portions; and wherein the drain grooves in said keeperportions are arranged so as to be between said bearing pads and saidsealing structures.